1. Is the Fish Sold as "Shishamo" Really Shishamo?
According to data from the Hokkaido Fishery Experiment Station, most fish distributed under the name "shishamo" are actually Karafuto shishamo, known in English as capelin. They belong to the same smelt family but are a different species. [1] Dictionaries also note that products sold as "roe-bearing shishamo" are not the same species as the true shishamo. [9]
Karafuto shishamo (capelin) are widely distributed in the northern Pacific and Atlantic oceans. It is said that 20,000 to 30,000 tons are imported to Japan each year. [1] In contrast, the true shishamo is a species found only in Japan, along Hokkaido’s Pacific coast. [1]
The Japanese kanji for shishamo, meaning "willow leaf fish," is generally thought to come from the Ainu words "sus" (willow) and "ham" (leaf). According to legend, shishamo were created when a deity floated willow leaves into a river. [8] However, there are other theories about the name's origin, so the exact source is not settled. So, where does the shishamo found only in this part of Japan live during the year?
2. Where Do Young Shishamo Go After Hatching in the River?
Shishamo hatch in rivers around April and May. Newly hatched larvae are 6 to 9 mm long and immediately drift downstream toward the river mouth. [4] [5] After that, they are thought to drift in the coastal sea until early May, staying near the seabed at a depth of about 10 m (33 ft) at least until July. [5]
Young fish prefer shallow waters. A report investigating shishamo juveniles off Kushiro in July found that at depths deeper than 10 m, nets caught juveniles only once out of 12 hauls. At depths shallower than 10 m, they were caught in 15 out of 18 hauls. [6] In the mouth area of the Mukawa River, juveniles were found in water no deeper than 9 m (30 ft). [6] The water temperature where juveniles were caught was 9 to 16 °C (48 to 61 °F). This survey was conducted in July and does not represent their location all year round.
As they grow, they move to slightly deeper waters. It is thought that they grow while moving between shallower and deeper areas, mainly in coastal waters 20 to 30 m (66 to 98 ft) deep. [5] Even when fully grown, their range stays within 120 m (394 ft) depth along the Pacific coast of Hokkaido. [3] [2] Thus, shishamo spend about 1.5 years in the sea. What triggers their return from the sea to the river?
3. Why Do Adult Shishamo Return to Their Birth River in Autumn?
Shishamo that have grown in the sea for about 1.5 years gather in river mouth areas in October and November, then swim upstream. [2] In the Mukawa River, upstream migration is seen from October to December, with a peak in mid-November. [7]
There is an order to the migration. Males enter the river before females and wait for them. Both males and females begin swimming upstream from the largest individuals. [7] One report states that the number of eggs per female increases by about twice for every 2 cm (0.8 in) increase in body length. [7]
The ovaries weigh about one-fifth of the female’s body weight. The ovaries of females just before migration are said to reach about 22% (one-quarter to one-fifth) of their body weight. [1] For a 100 g female, the ovaries would weigh 20 to 25 g. However, the figures used as a guide for migration vary by year, and a strong correlation has been seen with average coastal sea surface temperature from August to October. For example, in 2020, the day the value reached 19.6 was predicted as the start date, which matched the actual start of migration on November 9. [4]
Spawning grounds are meandering areas where seawater does not reach. The main spawning grounds in eastern Hokkaido are in seven rivers, including the Tokachi, Akan, and Kushiro rivers. All are meandering sections 1 to 10 km (0.6 to 6.2 miles) from the river mouth, unaffected by seawater influence. The spawning period is from mid-November to early December. [2] Where do shishamo go after spawning?
4. What Happens to Shishamo Parents After Spawning?
After spawning ends, males die in the river. On the other hand, some females go down the river as "downstream shishamo" and return to the sea. [8] [2]
When females caught in the coastal area of the Mukawa River immediately after spawning were examined, their stomachs contained crustaceans like shrimp and crabs, as well as fish larvae. [8] This study suggests that females, who used up energy for spawning, are eating in the sea to regain their strength. It is explained that whether these females will spawn again next year is still being investigated. [8] There may be more to the story beyond the 1.5-year cycle.
5. How Do Fishing Rules Protect Shishamo Parent Fish?
Shishamo fishing targets parent fish that have gathered in the coastal area near the river mouth for spawning. This means catching fish that are supposed to swim up the river and lay eggs, just before they do so. Therefore, fishing is adjusted by setting a goal for how many parent fish must be left to swim up the river. For the Mukawa River, the goal is to let more than 600,000 fish swim up. [8]
Fishermen made their own agreements. Fishermen in the Iburi and Hidaka regions formed a shishamo fishery promotion council and voluntarily reduced fishing by 30% of the permitted boats. They all take Sundays off. [1] In 2023 and 2024, voluntary fishing bans were also implemented to recover the resource. [8]
Catch amounts have changed over time. Catches were 1,000 to 4,000 tons per year in the 1960s, but dropped sharply in the 1970s, reaching 377 tons in 1988. Although they recovered afterwards, catches have been under 1,000 tons since 2011. [1] [3] Figures vary by area and year; in 1988, the catch in eastern Hokkaido alone was 224 tons. The population west of Cape Erimo is designated as a Local Population (LP) on the Ministry of the Environment's Red List 2020. [3] By leaving parent fish to return to the river, the shishamo's journey continues to the next year.
6. How Can You Check Shishamo Labels and Maps Yourself?
If you see a package of "shishamo" at home or in a shop, check how the fish name and place of origin are written. Check whether the package identifies the fish as capelin.
Look for the Mukawa, Tokachi, and Kushiro rivers on a map of Hokkaido. You will see that all these rivers are on the Pacific side. Try tracing the path from the river to the sea, and back to the river, on the map.
The figures in this article are taken from materials by the Fishery Experiment Station of the Hokkaido Research Organization. You can trace the original materials from the list of sources.